1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) optical network, and, more particularly, to an optical transmitter used in a WDM optical network, and a passive optical network using the optical transmitter.
2. Description of the Related Art
In WDM passive optical networks (PONs), particular wavelengths are assigned to respective subscribers each having an optical network unit (ONU). Accordingly, such a WDM PON ensures communication security, while being capable of easily accommodating a separate communication service required by a subscriber. Furthermore, PONs allow for the expansion of a subscriber communication capacity and it is also possible to simply increase the number of subscribers by assigning new wavelengths to new subscribers.
Generally, WDM PONs use a double star type topology. In the double star type topology, a remote node is installed in an area where a plurality of subscribers are distributed near one another. The remote node is connected to a central office via a single feeder fiber. The ONU of each subscriber is connected to the remote node by an independent distribution fiber. A multiplexed signal of downstream optical signals from the central office is transmitted to the remote node via the feeder fiber, and then demultipexed by an arrayed waveguide grating (AWG), for example. Thereafter, the downstream optical signals are transmitted to the individual ONUs via the respective distribution optical fibers. Upstream signal channels, i.e., wavelengths, outputted from respective ONUs are transmitted to the remote node, and then multiplexed by the AWG of the remote node. The resultant multiplexed signal of the upstream signal channels, i.e., wavelengths, is transmitted to the central office.
Recently, spectrum-sliced light sources have been actively researched for a wavelength division multiplexing light source. Such a spectrum-sliced light source slices incoherent light having a sufficiently wide wavelength band flat profile, using an optical filter or AWG, to provide a large number of wavelength-divided channels. In this case, it is thus unnecessary to use individual light sources, each respectively having particular oscillation wavelength, and a corresponding wavelength stabilizing device. For such a spectrum-sliced light source, a light emitting diode (LED), a superluminescent diode (SLD), a Fabry-Perot (FP) laser, a fiber amplifier light source, a picosecond pulse light source, etc. have been proposed. For example, injection light of a broad band generated from an incoherent light source such as an LED or fiber amplifier light source may be spectrum-divided using an optical filter or AWG and the resultant spectrum-divided injection channels, i.e., wavelengths, are provided to a reflective semiconductor optical amplifier, which is not provided with any isolator. Thus, the amplified light in the individual channels may be used for transmission of optical signals.
FIG. 1 is a block diagram illustrating an optical transmitter used in a typical PON. FIG. 2 is a diagram depicting waveforms of injection light A, and the signal light B shown in FIG. 1. As shown in FIG. 1, the optical transmitter 100 includes a broadband light source (BSL) 110, a circulator (CIR) 120, an AWG 130, N reflective semiconductor optical amplifiers (RSOAs) 140-1 to 140-N.
The broadband light source 110 outputs injection light A having a flat profile in a sufficiently broad wavelength band into N light beams of wavelengths λ1 to λN. (FIG. 2).
The circulator 120 has a first port 120-1 connected to the broadband light source 110, a second port 120-2 connected to a multiplexing port MP of the AWG 130, and a third port 120-3 connected to a transmission link. The circulator 120 receives the injection light A at the first port 120-1, and outputs the injection light A to the second port 120-2. The circulator 120 also receives signal light B at the second port 120-2, and outputs the signal light B to the third port 120-3.
The AWG 130 has N demulitplexing ports DP1 to DPN, in addition to the multiplexing port MP. The demulitplexing ports DP1 to DPN are connected to the RSOAs 140-1 to 140-N, respectively. For example, the N-th demultiplexing port DPN is connected to the N-th RSOA 140-N. The AWG 130 spectrum-slices the injection light A inputted to the multiplexing port MP, and outputs the resultant light beams to the demultiplexing ports DP1 to DPN, respectively. The AWG 130 further multiplexes the signal channels, i.e., wavelengths, inputted to the respective demulitplexing ports DP1 to DPN, and outputs a resultant multiplexed signal to the multiplexing port MP. The AWG 130 has wavelength transmission characteristics having periodically repeated free special ranges (FSRs). The AWG 130 has N wavelengths in an arbitrary FSR thereof. That is, the FSR has transmission wavelengths respectively corresponding to the N wavelengths.
The first through N-th RSOAs 140-1 to 140-N receive the first through N-th injection signals on the N channels and output first through N-th signal channels. For example, the N-th RSOA 140-N receives the N-th injection channels, amplifies the injection signal, and outputs the N-th injection signal, which has an increased peak power level. In this case, the N-th signal channel has an N-th wavelength.
In the above-mentioned optical transmitter, however, the injection light outputted from the broadband light source exhibits loss caused by mismatching of spectrums of the AWG and spectrums of the insertion light as well as insertion loss while passing through the AWG because it has a wide and flat profile. Such loss may be in the order of 3 dB. Hence, there is a need in the industry for a means to optically multiplex/demultiplex optical signals without incurring such loss.